Bubble column reactors (BCRs) are widely-used in the chemical industry as a low-maintenance, inexpensive means to mix and react gases with liquids, particularly in liquid phase oxidations of organic chemicals. In such liquid phase oxidations most, if not all, of the oxidation reaction occurs with oxygen dissolved in the liquid rather than the oxygen in the gas bubble. Therefore, the rate of oxygen dissolution is generally a prime factor in the process.
In its simplest form, a BCR as used for liquid phase oxidations is comprised of a column of liquid into the bottom quarter of which a reactant gas such as air or oxygen enriched air (e.g. air having up to 40 vol. % oxygen) is injected. The buoyancy of the injected gas causes the bubbles to flow upward. This upward flow of gas bubbles pulls surrounding liquid upward. The amount of liquid which flows upward due to bubble buoyancy exceeds the net liquid flow through the column. Therefore, the upward flow of liquid in regions where many or large bubbles exist must be countered by the downward flow of liquid in regions where such bubbles are rare. In this way, a liquid circulation pattern is created which is unique to the specific geometry of the BCR. Since BCRs have no mechanical agitation, the injected reactant gas functions to mix the liquid.
Most liquid phase oxidations of organic chemicals occur via free radical chain mechanisms. In general, the mechanism proceeds by four steps: initiation, propagation, branching, and termination. The termination of the radical chain involves the combination of free radicals. High molecular weight by-products are produced in termination reactions because two radicals react with each other prior to reacting with oxygen. The normal termination reactions are as follows: EQU ROO*+ROO*.fwdarw.R.dbd.O+ROH+O.sub.2 1) EQU R*+R*.fwdarw.R--R 2)
In the above reactions ROO* are peroxide radicals and R* are hydrocarbon radicals. In the presence of sufficient oxygen, the concentration of ROO* radicals is relatively high and the reaction of two ROO* radicals in reaction 1 dominates. However, if there is insufficient oxygen, as in an oxygen-starved region of a reactor, the reaction of the two radicals in reaction 2 will become significant, thus forming undesirable high molecular weight by-products. In addition to forming these by-products, the recombinations will enhance the termination rate over the propagation rate, consequently lowering the overall reaction rate.
The high molecular weight by-products are typically colored and difficult to remove from the final product. As such, they may devalue the product, even at very low concentrations. Thus the elimination of the radical pathway of reaction 2 can substantially increase product value and, in some cases, may also significantly increase reaction selectivity.
In reactors of this type the use of air enriched with up to 40% oxygen has been employed to increase production rates. However, the use of oxygen enriched air can create regions of the reactor having undesirably high reaction rates, and often undesired, excessively high, temperatures. Indeed this is often a problem for air fed reactors generally. Unfortunately, these high temperatures may promote the formation of byproducts such as carbon oxides in these regions, and as such the yield of the desired product and/or the productivity of the reactor is reduced.
Another problem with BCRs is that because of the flow patterns established, gas is not uniformly distributed in the liquid. Further, in the air-based oxidation of organic chemicals, oxygen-depleted air bubbles dominate large portions of the reactor. Coalescence of both these bubbles and feed air and/or enriched air bubbles leads to the formation of plumes of large bubbles which, due to their size, are very inefficient in transferring oxygen. Thus, even though oxygen may appear in the waste gas stream, the reaction may, in fact, be oxygen-deficient. In practice, due to inefficient mass transfer, only about 80% of the oxygen provided in either air or oxygen enriched air is typically utilized in the oxidations. Unfortunately, the remaining oxygen collects in the head-space of the reactor and may create an explosion hazard.
In some BCR systems, the regions where feed air or enriched air reacts are intentionally kept at an excessive temperature in order to ensure reaction prior to coalescence of the feed bubbles. The reason for such operation is to promote oxidation and keep the oxygen concentration in the waste gas stream below the explosive limit. Unfortunately, operation at such temperatures also may promote the formation of undesirable byproducts, such as carbon oxides in these regions, and the yield of the desired product and/or the productivity of the reactor is reduced. We should note that by the term "explosive limit" we mean the oxygen concentration at which the gas stream could be subject to explosion. Such limits differ depending upon reactant and process conditions, but are known to those skilled in the art.
Other practitioners have attempted to redistribute the gas feed at several stages in the reactor by breaking up coalesced bubbles such that the surface area for oxygen mass transfer is increased. Methods for redistribution include the use of perforated trays and/or packing materials. Each of these options has some disadvantages. For example, in addition to adding complexity to the reactor, they also add metal surface area. In most radical reactions, this is undesirable since undesired radical recombination is promoted at metal surfaces. Also, the presence of hardware in the reactor will substantially alter the circulation pattern and may actually reduce reactor productivity. Thus there is a need in the art to provide a simplified, more efficient method for preventing the formation of byproducts in BCRs.